The warning, which replaces one issued last April, specifically cautions travelers of the dangers in 14 of Mexico's 31 states, including the popular spring break destinations of Baja California Sur, Guerrero and Nayarit.

"The state of Guerrero was the most violent state in Mexico in 2015 for the third year in a row, and self-defense groups operate independently of the government in many areas of Guerrero," the warning says of the state that is home to the popular beachside city of Acapulco. "Armed members of these groups frequently maintain roadblocks and, although not considered hostile to foreigners or tourists, are suspicious of outsiders and should be considered volatile and unpredictable."

Acapulco has taken over from the northern border city of Ciudad Juárez to become one of the centers of Mexico's bloody drug war. The city suffers from being a strategically located drug trafficking hub on Guerrero's Pacific coastal highway, while mass tourism simultaneously provides gangs with a profitable local market for drugs.

It is also unfortunate to be the largest city in Guerrero state, Mexico's prime location for opium production and one of the most violent regions in the country, notorious for the disappearance of 43 students in 2014 and a seemingly incessant wave of violence and social unrest.

In 2009, the city still attracted as many as 30,000 American spring breakers, but only two years later that number had dropped to barely 500.

Despite the travel warnings in places like Acapulco and Cabo San Lucas, Mexico's tourism industry is booming.

Millions of Americans visit Mexico each year - including more than 150,000 who cross the border every day, the State Department says. The Mexican government has dedicated substantial resources to protecting major tourist destinations, the State Department says, and generally these areas do not see the levels of drug-related violence and crime that are seen along the border or major drug trafficking routes.